The operation of current commercial aircraft is highly automated, with the mission and trajectories flown being managed by a Flight Management System (FMS). Consequently, FMSs are programmed with flight plans generated by the aircraft operator, one of which will be chosen, adapted or input directly by the crew to be flown by the aircraft for the particular flight. The flight plans will normally have been designed in a manner to be advantageous to the aircraft operator from an economic perspective. Parameters such as climb, cruise and descent speeds, as well as operating altitudes, define the time of flight and fuel burn on the particular mission and these parameters are usually selected according to operational costs and other constraints (such as aircraft scheduling) in order to accommodate the aircraft operator's interests and needs. The flight plan will normally be submitted to the relevant Air Navigation Service Provider (ANSP) and is agreed upon prior to the start of the mission.
An aircraft, however, rarely flies according to the agreed flight plan without any alterations. This is because tactical variations from the flight plan invariably occur. Such variations may be due to factors such as operational delays, changes in aircraft operating weight, variations in weather conditions and new air traffic constraints. Factors such as delays, weight, air traffic control (ATC) constraints and winds normally effect the vertical profile of a flight plan, resulting in changes in the aircraft's speed and altitude schedules, whilst air traffic constraints and bad weather often also result in deviations in the plan path flown.
Tactical deviations from the flight plan often result in a penalty in terms of fuel burn, emissions and operating costs. The penalties arise from limitations in current technology and practice.
Airline operators often use what is referred to as the Cost Index (CI) to establish the preferred operating point (speed, time of flight and thus trip fuel burn) of a particular flight. The CI is an arbitrary parameter that relates time-related costs with fuel costs. By selecting a particular CI, the operator will be selecting the time of flight and one programmed in the FMS, the system will schedule the operating speeds and altitudes according to the operating weight and reported winds and temperatures entered into it. Due to the nature of the concept, the CI is often set for a particular flight or group of flights and is often not altered during their progress. This, naturally, may result in the aircraft effectively being flown less efficiently than possible.
Whilst the CI is a useful tool that allows the operator to select advantageous operating points of the aircraft, current methods that generate flight profiles based on this concept are limited in their ability to identify the most advantageous flight profiles that need to be flown in the prevailing conditions operating conditions. This is due to a number of factors, including the limited processing power on current technology flight management systems, which results in methods used being simple and approximate in nature.
ATC constraints may also introduce operational and cost penalties. ATC is primarily concerned with ensuring safe separation between aircraft and, accordingly, issue tactical instructions that constrain the flight path of aircraft when this is necessary. While ATC may be sensitive to expeditious routing, it does not explicitly take the operational costs of airlines into account. Tactical instructions, such as lateral deviations in the planned path and speed and altitude constraints are issued that, with current technology, do not allow sufficient time for the re-planning of the flight by the aircraft operator or pilot in a way to reduce the impact of these instructions on the operating costs of the flight.